The synthesis of hybrid organic-inorganic materials that combine the diversity and ease of processing of organic polymers with the thermo-chemical stability and oxidative resistance of ceramics remains a goal of material researchers worldwide. Polyhedral Oligomeric SilSesquioxanes (POSS) have emerged as effective multi-functional, highly tailorable additives, capable of improving polymer performance. Each of these nanoparticles features an inorganic SiO1.5 core, as well as an organic corona, which helps to determine overall solubility. Compounds based on this architectural framework have received a great deal of attention as nearly ideal hybrid materials due to the synergy of the silsesquioxane cage and organic character at the molecular level.
In contrast to most other forms of nanoscale reinforcement, POSS compounds have been shown to improve processing characteristics when either blended into polymer hosts or incorporated by copolymerization. Although both techniques have distinct advantages, inert blending is generally the preferred method, offering facile modification of commercial polymers without the necessity for polymer synthesis and balancing stoichiometry. Furthermore, blending techniques generally provide access to a greater material design space in the context of nanoparticle assembly. However, the availability of thermally stable, inert POSS additives for the purpose of reinforcing high temperature polymers by this method is limited. Further development would be of benefit to a range of potential products requiring lightweight materials for energy efficiency, aerospace, and durable infrastructure applications.
Aryl-functionalized silsesquioxanes, such as phenyl8Si8O12 (Ph8Si8O12), have been in existence for decades, appearing well suited for the preparation of high-performance, aromatic nanocomposites. However, the high symmetry and low dipole moments of Ph8Si8O12 promote highly-efficient crystalline packing. This is manifested in poor solubility in organic solvents, and a neutral response to mechanical shear, thus severely limiting incorporation into polymers.
To circumvent these limitations, several research groups have focused on the modification of aryl-functionalized POSS compounds. For example, vinyl8Si8O12 was functionalized with aromatic photo-luminescent compounds via Heck coupling. (Para-iodophenyl)8Si8O12 was synthesized as a platform for coupling additional organic moieties to POSS cages. Other work attached Ph8Si8O12 to polybenzimidazole via an in-situ Friedel-Crafts acylation copolymerization reaction. Researchers have found that copolymerized Ph8Si8O12 was more thoroughly dispersed in the polymer host than physically blended Ph8Si8O12.
Unfortunately, none of the above methods have proven to be fully successful in overcoming the limitations of non-reactive, aryl-functionalized POSS compounds such as poor solubility in organic solvents, and a neutral response to mechanical shear. There remains a need for aryl-functionalized POSS compounds which overcome these limitations.